mail6.verisign.com has no CNAME records.
Question and response
Question
dig mail6.verisign.com. NS
Answer
The response has no answer records.
Authority
verisign.com. 10800 SOA av1.nstld.com. mdnshelp.verisign.com. 1772110955 300 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0